In the field of oil and gas exploration and production, various tools are used to provide a fluid seal between two components in a wellbore. Isolation tools have been designed for sealing an annulus between two downhole components to prevent undesirable flow of wellbore fluids in the annulus. For example, a packer may be formed on the outer surface of a completion string which is run into an outer casing or an uncased hole. The packer is run with the string to a downhole location, and is inflated or expanded into contact with the inner surface of the outer casing or openhole to create a seal in the annulus. To provide an effective seal, fluid must be prevented from passing through the space or micro-annulus between the packer and the completion, as well as between the packer and the outer casing or openhole.
Isolation tools are not exclusively run on completion strings. For example, in some applications they form a seal between a mandrel which forms part of a specialized tool and an outer surface. In other applications they may be run on coiled tubing, wireline, and slickline tools.
Conventional packers are actuated by mechanical or hydraulic systems. More recently, packers have been developed which include a mantle of swellable elastomeric material formed around a tubular body. The swellable elastomer is selected to expand on exposure to at least one predetermined fluid, which may be a hydrocarbon fluid or an aqueous fluid. The packer may be run to a downhole location in its unexpanded state, where it is exposed to a wellbore fluid and caused to expand. The design, dimensions, and swelling characteristics are selected such that the swellable mantle expands to create a fluid seal in the annulus, thereby isolating one wellbore section from another. Swellable packers have several advantages over conventional packers, including passive actuation, simplicity of construction, and robustness in long term isolation applications.
FIG. 1 illustrates a swellable packer 100 according to the prior art formed on a tubular body 110 such as a mandrel having a longitudinal axis L. The packer 100 comprises a swellable element 120 disposed about the body 110. The swellable element 120 is formed from an elastomeric material selected to expand on exposure to at least one predetermined fluid. Such materials are known in the art.
The dimensions of the packer 100 and the characteristics of the swellable material of the swellable element 120 are typically selected such that the swellable element 120 forms a seal in use, which substantially prevents the flow of fluids past the body 110. On exposure to a wellbore fluid in the annulus surrounding the body 110, such as a hydrocarbon fluid, the swellable element 120 swells and its outer diameter increases until it contacts the surface of the wellbore to create a seal in the annulus. The seal prevents flow of fluid in the wellbore annulus between a volume above the packer 100 and a volume below the packer 100. Swellable packers are suitable for use in uncased hole and in a cased hole, in which case the swellable element 120 would form a seal against the interior surface of the outer casing.
Typically a packer will be constructed for a specific application and incorporated into a casing string or other tool string by means of threaded couplings, not shown in FIG. 1. In addition, although held in place by a simple offset in the example packer 100 of FIG. 1, the swellable element 120 may be held in place by gage rings or other attachment devices at either longitudinal end of the swellable element 120.
Swellable packers such as illustrated in FIG. 1 may be made with swellable elements 120 of various lengths to suit the application, typically ranging from 1 foot to 30 feet in length, although shorter or longer lengths are known. Furthermore, although a single swellable element 120 is illustrated in FIG. 1, packers 100 may employ multiple swellable elements 120 as desired.